Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in Graphene

TL;DR

This paper demonstrates ultrafast population inversion and optical gain in graphene using femtosecond pulses, revealing a transition from hot classical to dense quantum states within 200 femtoseconds.

Contribution

It provides the first experimental evidence of femtosecond-scale population inversion and stimulated emission in graphene, supported by a detailed theoretical analysis.

Findings

Broadband inverted Dirac fermion population forms within 35fs.

Negative optical conductivity and stimulated emission occur within 200fs.

Transition from hot classical gas to dense quantum fluid observed with increasing excitation.

Abstract

We show that strongly photoexcited graphene monolayers with 35fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative optical conductivity for the first 200fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.